Irish citizen and UK citizenship

Hi all,

My father is an Irish passport holder, and he has asked me about the possibility of applying for a UK passport on the basis of his mother being born in Ireland in 1906, pre. Republic. They moved abroad in 1907, and she was no longer living in Ireland at the time of the establishment of the Irish Free State.

From what research I could do I don't see that it is possible, something about the British Citizenship act in 1983 (?) and the status being non-transferable, but I wanted to consult with more knowledgeable people here first.